Lime is produced by the calcination of limestone by the removal of CO2 from CaCO3 to produce CaO. The limestone is generally preheated, prior to feeding the same to a kiln, such as a horizontally disposed rotary lime kiln, by passage though a vertically arranged preheating unit through which hot off-gases from the lime kiln flow, in a countercurrent manner. The hot off-gases, contain carbon dioxide and sulfur compounds that result from any sulfur compounds originally present in the limestone charge or in the fuel that is combusted to heat the charge in the rotary kiln. These sulfur compounds and solid particulate sulfur compounds are carried by the off-gases from the kiln through the limestone charge in the preheater, while off-gases preheat the limestone charge.
Because the fine solid particulates leaving the rotary kiln and fed to the preheater contain sulfur compound, and the finer sized solid particulates appear to contain the higher sulfur percentage, it is desirable to remove the solid particulates from the off-gases. Such solid particulates, when passed through the preheater with the hot off-gases can collect on the limestone charge and be recycled to the rotary kiln and increase sulfur compounds in the lime product discharged from the kiln, and can interfere with the preheater operation by causing plugging of the preheater.
In view of such problems, efforts have been made to remove the fine particulates or dust from the hot off-gases of a rotary lime kiln prior to passage through the preheater. Current practices for such removal of particulate material involve the use of screens or by-pass systems.
A problem with the use of screens is that they remove too large of a dust fraction and the finer (higher sulfur portion) remains in the kiln system. The screen deck also warps from the kiln gas temperature, accumulates dust build-up and blinds over. Recent designs use mechanically driven dynamic screens in an attempt to prevent the deck from blinding. These screens are complicated, costly and difficult to maintain. Retrofitting a screen onto an existing preheater is also extremely difficult and costly.
By-pass or stripping systems use ducts or probes with ambient cooling air and an external high-temperature fan to remove some hot kiln gases and dust from the preheater transfer chute. The dust is separated from the gas in an external cyclone and the cooled kiln gases are returned to an air pollution control device. Problems include build-up and plugging of the probe and ducts, additional electrical power usage by the external fan, and the fuel efficiency of the kiln suffers because some hot kiln gas by-passes the preheater. The system is also complicated and costly. An additional air pollution device is required for the by-pass gases if the original device is not large enough.